Many common building materials, such as concrete, gypsum, brick, certain types of stones, and many insulating materials have a capillary pore system. These pore systems frequently can become filled with water, particularly where the materials are in contact with a moisture source, such as moist soil or the like. In many cases, a long term continuation of such moisture-saturated conditions in the building material provides an undesirable environment and/or the potential for deterioration of the materials.
Known procedures for drying saturated, porous materials have proven to be less than satisfactory. One of the traditional methods is the use of a combination of heating and ventilation. However, not only are such procedures very slow, they also utilize large amounts of energy. Further, there is an attendant risk, with any process utilizing heat, of thermally induced warping and/or cracking of the structure.
Another known technique for eliminating water from porous materials is electro-osmosis. Thus, it is known that the walls of the capillaries in most common building materials are covered with an electrically charged, adsorbed water film, sometimes referred to as an electrical double layer. It has been established that, if such a porous body is subjected to an electrical field, part of the so-called double layer will tend to migrate under the influence of the field. Some of the free liquid in the pores is carried along in this process, which can lead to a significant reduction in the internal moisture content of the porous body.
Notwithstanding the theoretical attractiveness of the electro-osmosis process, there have been serious drawbacks to its use in actual practice. One of the reasons for this is the extremely low efficiency experienced by conventional systems. In this respect, the application of an electric charge to a wall of porous building material typically involves the use of electrodes provided in or installed in the porous material, and connected through the material to a grounding electrode. When the electrodes are polarized, there is a migration of water molecules toward the negative electrode. However, after the system has been in operation for a period of time, the electrodes become covered with coherent films of gas, commonly hydrogen gas at the cathode, and in other cases with oxide, sulfide or other films formed by electrochemical reactions at the electrode surfaces. These films have very high electrical resistance, which leads to a deterioration in the electrical characteristics of the system and results in low operating efficiencies.
Another important problem encountered with conventional electro-osmosis practice arises from the fact that the positively charged electrodes of the system are subject to a high degree of electrolytic corrosion. Where the electrodes are installed especially for the purpose, such corrosion results initially in a reduction in efficiency of the system and, eventually, in complete electrical discontinuity at the electrode. In many cases, however, it is desirable to use the internal reinforcing steel as the positive electrode. In such cases, heavy corrosion of the positive electrode can be significantly degrading to the structure itself.
It has been proposed (for example in Swedish Patent Application No. 86/01888-4, filed Apr. 24, 1986), to utilize bursts of rapidly alternating asymmetrical wave forms in an effort to achieve electro-osmosis. Such a procedure is believed to be altogether impractical, however, because the extremely high rate of cycling causes significant power loading and high RFI radiations.
In accordance with the present invention, a novel and improved system and procedure is provided for carrying out electro-osmotic drying of porous structures in a manner to obviate the problems normally arising from polarization of the electrodes and the concomitant loss of operating efficiency and/or deterioration of the system, or possibly even of the structure itself. The present invention has particular relevance to foundation walls and similar structures. It is particularly useful, for example, in reducing the pore liquid in damp cellar walls. Other structures with which the invention can be usefully employed are concrete water tanks, retaining walls, bridge decks, structural columns, etc. In general, the invention can be usefully employed in connection with any structure of concrete or other porous building material, which is exposed to water and subject to degradation by capillary action.
Certain aspects of the invention are also useful to advantage in the re-alkalization of reinforced concrete that has become acidified through carbonation.